The present invention generally relates to a method for use in fabricating and/or testing a thin mirror and, in particular, relates to one such method including the step of interfacing a rigid support structure with the thin mirror such that stress and strain in the mirror are avoided.
In the field of optics, there is an ever increasing demand for larger and larger diameter mirrors. One reason for this demand is that, as is well known in the field of astronomy, the distance for which a telescope is practical is dependent upon the amount of light the telescope mirror can capture. Hence, one approach for capturing more light in a telescope is to increase the size of the mirror. However, as the size of the mirror is increased, significant problems occur.
One problem that must be addressed is that the mass of the optic significantly increases with even relatively small increases in the diameter. One approach to compensate for this problem has been to reduce the thickness of the mirror to thereby reduce the mass of the optic. Such an approach, however, introduces additional difficulties that can often counter the advantages achieved by the reduction of mass. One such difficulty is that as the optic is made thinner, it becomes more susceptible to deformation from its own weight. That is, a thin mirror can develop optical surface deformations simply because portions of the optic sag under its own weight.
Typically, the more conventional approach to addressing the problem of such deformations has been to fabricate the optic in relatively thick rigid segments and, when ready for use, arrange or assemble the segments to align the optical surfaces. The premise of this approach is that each segment can be fabricated to avoid such mass to thickness difficulties. Usually when the optic is assembled the segments are independently shifted to provide the desired optical surface. In many instances, the position of each segment is controllable by actuators that contact the rear of the segment and which can move the segment as needed to provide orientation and alignment of the front optical surface.
However, the arrangement of segments to form a high quality optical surface is quite expensive and introduces considerable complexity to the design of the optic. In addition, such an approach carries a substantial weight penalty that is very undesirable for mirrors to be launched into space.
Another approach to resolving the mass to thickness difficulties that are inherent in large thin optics is to shape the optic as a single piece and employ actuators to drive the optic to the desired shape.
To date, this latter approach has exhibited some particular drawbacks that can cause the catastrophic loss of an entire optic. One such drawback stems from the fact that, during conventional fabrication, internal stresses and strains are introduced into the optic. Such internal stresses and strains can cause the optic to crack or fracture either during the fabrication process of the optic or when actuators are activated during the use of the optic.
Another drawback is that during the fabrication process deformations of various spatial frequencies that cannot be removed by actuators can be introduced. For example, deformations having high spatial frequencies, i.e., where the surface deformations are between the actuators, cannot be corrected by the actuators.
A further difficulty in the fabrication of thin optics is that the optic, even during fabrication, is quite flexible. This flexibility makes it very difficult to fabricate and certify these thin optics to fight geometrical tolerances. Any geometrical differences from the final desired shape usually means that the optic is either stressed, strained, or both.
Currently, such thin optics are fabricated by initially generating a very rigid block of granite so that the surface thereof matches the rear surface of the optic to be formed. The generated surface of the block of granite is intended to match the rear surface of the optic in a strain free manner. The optic is then attached to the granite with some form of adhesive, such as, pitch. In reality, the rear surface of the optic rarely truly matches the front surface of the block of granite. As a result of the mismatch additional strain is introduced into the optic during the optical surface finishing process. Conventionally, after adhering the optic to the block, the surface thereof is generated with the optic in the strained condition. The optic is then usually tested on the granite block and then again tested on a fixture that attempts to secure the optic without introducing additional stress or strain. Thereafter, the optic is installed in an optical system that includes, among other components, a plurality of actuators to forcibly adjust the surface into the desired configuration. It is readily understood that, if the optic has suffered internal stresses and strains during fabrication, the forces exerted by the actuators can cause the optic to fracture and/or have an optical figure that is beyond the range of being corrected.
In addition, there is usually at least one process step during the manufacturing process that involves subjecting the optic to temperatures greater than room temperature. Such a step almost inherently creates a built in strains in the optic when the optic is returned to room temperature.
Consequently, it is highly desirable to develop a method for use in fabricating and/or testing a thin mirror that, in fact, reduces the stresses or strains introduced into the optic material.